This invention relates to an adaptive predictive encoder for use in a predictive code communication system.
A predictive code communication system is suitable in effectively transmitting and receiving highly redundant signals, such as a television signal. A typical one is a differential pulse code modulation system (DPCM). According to simplest differential pulse code modulation of a signal, a difference (called a predictive error signal) between a present or current sample of the signal and a predicted value therefor is encoded to produce an information code with the predicted value being given by decoding another information code produced for a next previous sample. This system is very effective when applied to a television signal. The predictive error signal, however, has an amplitude distribution concentrated in a small amplitude region so that the information codes for smaller quantization levels appear more frequently to result in another redundancy. It is therefore possible to further raise the efficiency of encoding by adopting variable length codes with an information code of a shorter duration assigned to more frequently occurring quantization levels.
For differential pulse code modulation of a television signal, variable length codes make it possible to decrease the amount of information by one bit per sample on the average as compared with equal length codes. In practice, the decrease varies from zero to about two bits depending on the nature of the signal. A buffer memory is therefore used to temporarily store the variable length codes before transmitting them at a predetermined transmission or bit rate. Overflow of the buffer memory is nevertheless inevitable for a signal portion carrying much information, namely, for rapidly varying information, when use is made of a transmission rate suitable to an average amount of the information.
In order to avoid the overflow, proposals have been made to somehow change the quantization characteristics when the buffer occupancy tends to increase beyond a certain threshold value. For example, the quantization levels may be restricted either to prohibit production of longer codes or to switch the longer codes to equal length codes. The restriction imposed on the number of quantization levels, however, undesiredly strengthen the overload distortion to degrade the encoding performance. On encoding a television signal, this leads to ambiguous outlines of pictures or production of the so-called edge business. Alternatively, the quantization characteristics may be changed by widening the amplitude range of the samples for the smallest quantization level, such as the zero quantization level, so as to make shorter codes appear more frequently. A wider quantization level for small amplitude samples, however, results in a greater quantization error.